From the food he consumes, a man develops horsepower, but only one-tenth is available for sustained useful work. The remainder is expended in life functions of the body. Thus, as an engine converting fuel to mechanical energy, we are about 20 percent efficient. Our muscles do provide some work overload reserve. For bursts of energy of less than a second, up to six horsepower may be expended. A ■ Jle of thumb for useful power for periods of four minutes to eight hours is mathematically expressed by:
For example, a man can put out one-eighth horsepower over four hours, but if he is putting out one-fourth horsepower he will last only about 12 minutes. The above rule of thumb assumes a 35-year-old male European laborer. A 20-year-old male will be able to generate 15 percent more useful work while a 60-year-old will generate about 20 percent less.
Man cannot compete successfully with engines or dumb animals as a source of power for constant-load repetitive jobs such as pumping water for irrigation. His low power output places him at a disadvantage. However, man has a unique brain and is well adapted for jobs with a low power requirement but demanding dedsion-making and dexterity of hand. For example, transplanting rice demands thought as well as dexterity—does the plant have sufficient roots? Is the spacing correct?
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